Combating unwanted vegetation with 2-aryl-5-substituted 1,3,4-oxadiazoles

ABSTRACT

UNWANTED VEGETATION IS COMBATED BY APPLICATION OF A CLASS OF PHENYLOXADIAZOLES WHICH POSSESS EITHER OR BOTH PRE- AND POST-EMERGENT PHYTOXICITY. THE HERBICIDES AS A CLASS ARE SEVERELY TOXIC TO A MINORITY OF PLANT SPECIES, MOST OF THE COMPOUNDS HAVING PRE-EMERGENT EFFECTIVENESS AGAINST CRABGRASS. SOME OF THE COMPOUNDS, AS FOR EXAMPLE, 2-(3&#39;&#39;,4&#39;&#39;-DICHLOROPHENYL)-5-METHYL-1,3,4-OXADIAZOLE, ARE PARTICULARLY USEFUL FOR PRE-EMERGENT CONTROL OF WEEDS IN A STANDING CROP BECAUSE OF AN ALMOST TOTAL ABSENCE OF POST-EMERGENT PHYTOXICITY. A SMALL GROUP OF COMPOUNDS, SUCH AS 2-DIMETHYLAMINO-5-(2&#39;&#39;,4&#39;&#39;-DIMETHYLPHENYL)1,3,4-OXADIAZOLE ARE PREDOMINANTLY POSTEMERGENT HERBICIDES AND A NUMBER OF COMPOUNDS, AS FOR INSTANCE, 2-DIETHYLAMINO-5-P-TOLYL-1,3,4-OXADIAZOLE, HAVE BOTH PRE- AND POST-EMERGENT ACTIVITY AND ARE USEFUL IN COMBATING BOTH CRABGRASS AND BROADLEAF WEEDS IN FIELDS OF SMALL GRAINS AND IN BROME GRASS PASTURES.

United States Patent US. Cl. 260-307 G 1 Claim ABSTRACT OF THE DISCLOSURE Unwanted vegetation is combated by application of a class of phenyloxadiazoles which possess either or both preand post-emergent phytotoxicity. The herbicides as a class are severely toxic to a minority of plant species, most of the compounds having pre-emergent effectiveness against crabgrass. Some of the compounds, as for example, 2-(3,4'-dichlorophenyl)-5-methyl-l,3,4-oxadiazole, are particularly useful for pre-emergent control of weeds in a standing crop because of an almost total absence of post-emergent phytotoxicity. A small group of compounds, such as Z-dimethylamino-S-(2',4'-dimethylphenyl)1,3,4-oxadiazole are predominantly postemergent herbicides and a number of compounds, as for instance, 2-diethylamino-5p-tolyl-1,3,4oxadiazole, have both preand post-emergent activity and are useful in combating both crabgrass and broadleaf weeds in fields of small grains and in brome grass pastures.

DESCRIPTION OF THE INVENTION This is a division of application Ser. No. 869,369 filed Oct. 24, 1969, now US. Pat. 3,718,452.

Selective herbicides which are efiective to combat only a few particularly troublesome species or which may be' class of 2-phenyl-1,3,4-oxadiazoles having the structural formula in which R is selected from the group consisting of hydrogen, mercapto, thiocyano, amino, lower alkylamino, di(lower alkyl)amino, methyl, cyclopropyl, ethyl and propyl; R is selected from the group consisting of hydrogen, ehloro and methyl; R" is selected from the group consisting of hydrogen, chloro, nitro, bromo, iodo and methyl and R is selected from the group consisting of hydrogen, methyl, chloro, bromo and fiuoro substituents. These substances are generally phytotoxic to less than half of a group of test species representing about fifteen plant families. By virtue of this selective behavior they are useful for combating unwanted vegetation in the presence of seeds and living plants of other species.

The preparation and use of the novel herbicides are discussed below.

SYNTHESIS OF THE HERBICIDES The starting materials for most of the compounds employed as selective herbicides are aromatic acid hydrazides and the synthesis methods are summarized on the following flow sheet, Scheme A. Some of the compounds are also conveniently prepared from aryl tetrazoles, using the reactions shown in Scheme B.

3,808,223 Patented Apr. 30, 1974 SCHEME A NN l I CNBr I l Ar-L SH Ari 5 CN KOH CS2 0 RC(OEt)a NN CNBr i l Ar NHNH: or JL l M NH, NH-HCl Ar R 0 g 0 R -0Et /RNCO O I 1 R301 o o o o Ari JNHNlEH JNHR AriiHNHNi m NN Ml l...

SCHEME B l W i) IT Ar R Ar- R N 0 H N=I|-1 A N--N R I Ar-k /N N Ari LN/ Illustration of Scheme A The individual methods shown in Scheme A are specifically exemplified below for illustrative purposes.

Method 1 0 Ar NHNH: OS: KOH

O O i l A l Ar NHNH SK A1 0 SH Method 2 NN N-N Ar-l JSH KOH CNBr Ari SCN O 0/ Preparation of 2-thiocyano 5 (p-fluorophenyl)-1,3,4- oxadiazole.-To a stirred solution of 4.1 g. Z-mercapto- 5-(p-fluorophenyl)-1,3,4-0xadiazole and 1.2 g. potassium hydroxide in 50 ml. of methanol is added 2.2 g. of solid cyanogen bromide. The solution is stirred an additional half hour, diluted with water and the precipitate is filtered and washed with water, giving 3.5 g. of 2-thiocyano- 5-(p-fluorophenyl)-l,3,4-oxadiazole, M.P. 91-6.

Method 3 Method 4 NN 0 Ar lNHNHt lNHR Ar-i NHR Preparation of 2-propylamino (m-chlorophenyl)- 1,3,4-oxadiazole.A mixture of 16.7 g. l-(m-chlorobenzoyl)-4-propylsemicarbazide and 120 ml. of phosphorus oxychloride is refluxed 2 hrs.'Most of the solvent is distilled at reduced pressure and the residue is poured onto ice. Neutralization with concentrated NH OH gives 11.3 g. of crude solid, recrystallization of which from ethylene dichloride-hexane gives 4.0 g. of 2-n-propylamino-5- (m-chlorophenyl)-1,3,4-oxadiazole, M.P. 122.5-l23.0.

Method 5 o I Arl lNHNHz RC(0Et); ray-L in Preparation of 2-(3',4'-dichlorophenyl)-5-methyl-1,3,4- oxadiazole.-A mixture of g. 3,4-dichlorobenzhydrazide and -65 ml. triethyl orthoacetate is refluxed 18 hours. Forty milliliters of solvent is then removed by distillation at atmospheric pressure. The residue is cooled to 90, 75 ml. of hexane is added, and-on further cooling, 10.5 g. of 2-(3',4-dichlorophenyl)-5-methyl-1,3,4-oxadiazole, M.P. 147-9 crystallizes.

Method 6 O 0 ill g PO01: l I Ar NHNH R Ar 0 B.

Preparation of 2-cyclopropyl-5-(m-bromophenyl)-1,3,4- oxadiazole.A m'urture of 8.7 g. of N-(m-bromobenzoyl)-N'-cyclopropanecarbonylhydrazine and 100 ml. of phosphorus oxychloride is heated one and one-half hours at 85-90. The solution is concentrated to about 50 ml. by distillation at reduced pressure and the residue is poured onto ice, precipitating 7.1 g. of 2-cyclopropyl-5- (m-bromophenyl)-1,3,4-oxadiazole, M.P. 78-80.5.

Method 7 Preparation of 2-ethly 5 (m chlorophenyl) 1,3,4- oxadiazole.-To a solution of 17.0 g. of m-chlorobenzhydrazide in 50 ml. of ethanol is added slowly a solution of 13.7 g. of ethyl propionimidate hydrochloride in 40 ml. of ethanol and the mixture is warmed one hour on the steam bath, boiled down and allowed to crystallize, giving 5.9 g. of 2-ethyl-5-(m-chlorophenyl) 1,3,4 oxadiazole,

4 Method 8 Preparation of 1-methyl-4-(1,3,4-oxadiazol 2 yl)- pyridinium iodide.--A solution of 5 g. of 2-(4-pyridyl)-1, 3,4-oxadiazole and 7.3 g. of methyl iodide in 30 ml. of N,N-dimethyl formamide is stirred 4 hours at room temperature, then diluted with anhydrous diethyl ether, precipitating 9.3 g. of 1-methyl-4-(1,3,4-oxadiazol 2 yl)- pyridinium iodide, M.P.. 208-10". I

Illustration of Scheme B The aryltetrazoles employed as starting materials in Scheme B may be prepared by the method of W. G. Finnegan, R. A. Henry and R. L. Lofquist (J. Am. Chem. Soc., 80, 3908 (1958) and US. Pat. 2,977,372) which utilizes the reaction of an arylnitrile with a mixture of amonium chloride and sodium azide in dimethylformamide containing a catalytic amount of lithium chloride.

' Method 9 NN 0 01- I (03111 Preparation of 2-n-propyl-5-(3',4'-dichlorophenyl)-1,3, 4-oxadiazole.A solution of 5.0 g. of 5 (3,4 dichlorophenyl)-tetrazole in 50 ml. of butyric anhydride containing 10 drops of pyridine is refluxed 18 hours. Most of the excess anhydride is distilled at reduced pressure and the residue is crystallized from hexane, giving 3.0 g. of 2-npropyl-5-(3',4'-dichlorophenyl) 1,3,4 oxadiazole, M.P. -82.

Method 10 The Z-dialkylamino-S-aryl-l,3,4-oxadiazoles may be prepared from 2-dialkylcarbamyl-5-aryl tetrazoles. The required tetrazole intermediates are prepared by reacting an aryltetrazole with a dialkylcarbamyl chloride according to the following general procedure.

Preparation of S-aryl- 2 dialkylcarbamyl 2H tetrazoles.-To a stirred solution of the appropriate tetrazole dissolved in a minimum volume of pyridine is added dropwise the dialkylcarbamyl chloride. Keping the temperature below 30 by cooling the reaction flask in an icebath, the reaction mixture, after standing for a substantial period of time, as for example overnight, is cooled in the ice-bath and diluted with water. The insoluble material is collected by vacuum filtration and allowed to dry in air. The crude product may be purified by dissolving it in ethyl acetate, at room temperature, filtering the resulting solution and precipitating the product with hexane. The nuclear magnetic resonance spectrum of the product shows a characteristic broad peak for the hydrogen atoms attached to the carbon atom adjacent to the carboxamide nitrogen. The infrared spectrum of the product exhibits a peak at about 5.8 millimicrons. 

